A mortar may be arranged on a movable platform, such as on an armoured vehicle, whereby the mortar may be easily moved from one location to another, and on the other hand, it may be quickly moved to safety from an emplacement. If the mortar is to be fired horizontally or downward, a problem arises from the fact that the shell does not stay in place in the mortar barrel, but may slip forward in the barrel so that it can no longer be triggered. U.S. Pat. No. 5,503,080 discloses a support member that is fastenable by friction to the fins in the tail end of the shell. The solution taught by the patent has the problem that its application requires very exact measurements of both the tail end of the shell and the support member itself to be able to function at least fairly reliably.